From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Experiences and findings related to crankshaft vibrations in multi-crank engines.
| Identifier | ExFiles\Box 25\3\ Scan232 | |
| Date | 28th January 1927 guessed | |
| CRANKSHAFT VIBRATIONS. My experience has been wholly confined to multi crank engines and have found that critical speeds could be estimated within about 7% on such engines from a knowledge of the crankshaft stiffness and polar moment of inertia of the parts. The values for stiffness and inertia being determined experimentally. It was further found that assuming half the mass of the pistons concentrated at the respective crank pins and including this in the inertia of the crankshaft, gave good results where previous calculation neglecting the piston masses shewed that the calculated values fell short of the observed. Reference was made to the fact that tightening up the air screw raised the critical speed to a higher value. It would be interesting to know if the energy dissipated in friction by the loose air screw as shewn by the hysteresis curves - resulted in any appreciable reduction in the amplitude of the vibration at the resulting critical speed. The Austro Daimler Co. I believe, employ their flywheel frictionally mounted presumably as a means of avoiding torsional resonance within the working speed range. We once carried out a similar experiment in which the flywheel was frictionally clamped to the crankshaft and could slip a few degrees relative to the crankshaft against this friction before its movement was restricted by rubber buffers. This resulted in removing the critical speed from the working range but a good deal of 'thrashing' of the flywheels against the rubber buffers was evident at low speeds due to the engine torque fluctuation. I note from diagram 8, that when the flexibility of the shaft was increased to lower the synchronous speed from 1400 to 820 r.p.m. the torque peak of the vibration was considerably reduced. I understand that the 'resonance factor' is the same at each of these speeds and that this reduction of vibration amplitude is due to the smaller gas torque at the lower speed. This being the case it appears that no advantage could be gained by application of the same method to an automobile where the gas torque can remain fairly constant and even become higher at the low speeds. contd :- | ||